mimecast: check ttp URL source IP values for type conformity

[efd6]

What does this PR do?

Checks ttp URL source IP values for type conformity. Also fix field mapping name for sourceIp. This is not breaking since the currently used name, sourceIP would never have been set.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

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• Start Time: 2023-05-01T23:11:16.303+0000

• Duration: 19 min 34 sec

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Test	Results
Failed	0
Passed	56
Skipped	0
Total	56

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (8/8)	💚
Files	100.0% (8/8)	💚
Classes	100.0% (8/8)	💚
Methods	95.122% (117/123)	👎 -4.878
Lines	88.725% (1267/1428)	👍 3.635
Conditionals	100.0% (0/0)	💚

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[kcreddy]

LGTM 👍🏼

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Package mimecast - 1.8.1 containing this change is available at https://epr.elastic.co/search?package=mimecast